Seeking advice & suggestions on sandboxing/backups

[ChaosZen]

When creating a mod from scratch, using a fresh vanilla installation, what steps and methods do you use, or would you suggest, in order to have a clean vanilla install to start with, while not losing your previous games and installation configs, mods, save games, add-ons, and all else? Or, what other advice, and suggestions would you offer as first steps for preparing to create new mods? 

 

My personal interests are more along the lines of scripts and immersion, and not so much the more common new outfits, objects or buildings. So when you create a mod, how do you test it without having other mods or custom content possibly getting in the way?

 

If possible, I would like to hear some suggestions for both those who use MO, as well as those who use NMM as their mod manager.

 

To preserve previous installs and playthroughs in the past, I'd make a complete copy of my Skyrim folder and my NMM folder on a separate hard drive, but it can sometimes be confusing when losing track of which backups go where, because the different default file locations, with some things being saved to NMM folders, while others saved to Skyrim folders. What steps or practices do you use to keep playable backups in order?

 

If I had more computers which could handle the hardware load, it would be easy, simply keep one as-is for playing, while using a clean install on another as a sandbox to script or create in. Does anybody know of any reliable method for doing something similar on a single computer? I suspect that since Steam/... /Skyrim... is located in C:/Program Files (x86), that it would be pointless to even log in as a different user, since the folder location is globally shared.